A frequently conducted analytical procedure is the determination of the presence and/or relative amount of certain chemical compounds in samples in which, if the chemical compound is present, may be in a relatively small amount, e.g., in the parts per billion or less range. These analytical procedures may be conducted, for instance, to analyze water or soil for the presence and/or relative amount of organic pollutants such as aliphatic or aromatic compounds such as benzene, chlorobenzene, phenanthrene, anthracane, polychlorinated biphenyls, trichloroethane, naphthalene, and the like. In conducting these environmental analyses, because of the small amount of the analyte present and the possible deleterious effect of massive amounts of the medium containing the analyte on the analytical detection means, it is generally desired to concentrate the analyte or remove it from the medium or provide it in a suitable medium for the analysis. For example, soil samples cannot be directly used for mass spectroscopy or gas chromatography analyses. The large amount of water in water samples may overpower mass spectrometers or gas chromatographs.
Any analytical method for determining analytes in, e.g., soil or water samples should be repeatable, i.e., the analytical technique should not introduce undue sample-to-sample variations. Moreover, it should be relatively non-complex and require minimal time and effort to conduct. If, for instance, a plot of land is to be inspected for potential contamination, the greater the number of soil samples analyzed, the greater the assurance that an accurate representation of the condition of the land will be achieved. With more labor intensive, longer duration analytical procedures, analytical costs may become very significant, especially for large plots of land.
Because of the importance of analytical protocol in the arena of environmental evaluations, the United States Environmental Protection Agency has issued during September, 1986, Method 8240 entitled "Gas Chromatography/Mass Spectrometry for Volatile Organics". The scope of the method is stated to be useful
. . to determine volatile organic compounds in a variety of solid waste matrices. This method is applicable to nearly all types of samples, regardless of water content, including ground water, aqueous sludges, caustic liquors, acid liquors, waste solvents, oily wastes, mousses, tars, fibrous wastes, polymeric emulsions, filter cakes, spent carbons, spent catalysts, soils and sediments. (Page 8240-1). PA1 An inert gas is bubbled through the solution at ambient temperature, and the volatile components are efficiently transferred from the aqueous phase to the vapor phase. The vapor is swept through a sorbent column where volatile components are trapped. After purging is completed, the sorbent column is heated and backfiltered with inert gas to desorb the components onto a gas chromatographic column. (Page 8240-6). PA1 Organics in aqueous solutions are recovered through steam distillation. This allows the compounds to be volatilized and transported to the adsorbent tube at temperatures well below their boiling points. Thus, sample breakdown is prevented. To prevent water vapor in the effluent from condensing in the tube, dry gas is added to the distilled samples and the adsorbent tube is heated. This ensures efficient trapping of the organic molecules. PA1 Volatile, semivolatile, and nonvolatile organic contaminants in environmental sample matrices (water, wastewater, soil, etc.) are usually prepared for chromatographic evaluation through different techniques. Typically, volatile compounds are prepared by purge and trap methods, and semivolatile and nonvolatile compounds are prepared by liquid-liquid extraction. Multi-bed adsorbent tubes containing several nonspecific, carbon-based, Class I adsorbents have been constructed. These tubes effectively adsorb and subsequently thermally desorb the volatile, semivolatile, and nonvolatile fractions sparged (steam distilled) from aqueous sample matrices. When interfaced with a dynamic thermal stripper and a thermal desorber unit, these tubes permit simultaneous analysis of all three contaminant fractions. This adsorbent tube/instrumentation system eliminates the tedious sample preparation, and other shortcomings, associated with recovery methods incorporating a combination of purge and trap and liquid-liquid extraction. PA1 Nonspecific Class I adsorbents adsorb compounds according to molecular size and shape, thus eliminating concern over which functional group(s) an adsorbate possesses. Because these adsorbents are also hydrophobic, competition for the adsorbent surface favors the analytes of interest during steam distillation. Utilization of four Class I adsorbents, each having a different surface area, allows the adsorbent tube to function in a size-exclusion operating mode. These adsorbents include two graphitized carbon blacks and two carbon molecular sieves. PA1 The organic contaminants are steam distilled by using the thermal dynamic stripper. This unit transfers the organic contaminants from the sample matrix to the adsorbent tube by shifting the system equilibrium to favor the vapor phase. Following adsorption, the adsorbates are effectively transferred to a gas chromatograph via the thermal desorber unit. This unit interfaces with both packed and capillary columns, hence several analytical columns may be chosen. PA1 As is known, ultraviolet light facilitates and enhances the oxidation action of ozone and has an incidental heating effect that causes evaporation of some of the quantity of water, which is increasing by the development of water as a product of oxidation of the organic material.
Method 8240 contemplates the likelihood that analytes may have to be recovered from samples prior to being introduced into, e.g., a gas chromatograph. These methods include a purge-and-trap process:
For sediment/soil and waste samples, an extraction method, among others, is recommended. See, for instance, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, SW-846, Third Edition, United States Environmental Protection Agency.
In one currently practiced method for the determination of organic compounds in environmental samples, the original matrix (such as soil, water, sludge, etc.) is treated by liquid-liquid extraction with an organic solvent, e.g., methylene chloride, hexane, 2-propanol, cyclohexane or acetonitrile to remove organic compounds. The extraction is for a period of 18 to 24 hours. The sample may then be dried and subjected to further treatment by solvent exchange. See Method 3520, September 1986. Method 640, September, 1986, disclosed a gel-permeation technique for clean-up. Both the above Methods are contained in the above Environmental Protection Agency document. This extract can be introduced into the analytical instrument such as a gas chromatograph/mass spectrometer. The sample preparation procedure often takes up to 72 hours to complete.
It is not uncommon to encounter emulsions during an extraction procedure involving an aqueous sample. This may make difficult, if not impossible, separation of the organic phase and can result in a loss of sensitivity due to a partial loss of the organic compounds. Another problem that can be encountered occurs when the sample contains heavy organic residues. The extract will contain the organic residues and can lead to fouling of the analytical system. The fouled system must be cleaned to remove the residue build-up. Methods do exist for removing the heavy organic residue from the samples; however, such procedures require time and effort and may result in a partial loss of the organic compounds sought to be detected by the analytical system. One procedure for cleaning an extract containing heavy organic residues involves a gel-permeation technique such as disclosed in Method 3640 discussed above.
For purposes of gas chromatography/mass spectroscopy, analytes are generally placed into two categories: volatiles and semivolatiles. Current analytical protocols specify different procedures for analytes in each category. Hence, two sample preparations may be required to cover organic compounds in both categories.
Supelco Equipment Co. markets a Dynamic Thermal Stripper for purging volatile or nonvolatile organic compounds from samples for analytical evaluation. Page 240 of Supelco's 1989 Catalogue No. 27 bills the dynamic thermal stripper as "Replace Tedious Liquid-Liquid Extractions, Thermal Stripper Ensures Efficient Recovery of Many Compounds". The unit is depicted as having a sample vial through which a purge gas is bubbled. The sample vial is contained within a heated airbath oven (30.degree. to 200.degree. C.). The effluent from the sample vial is passed to a thermal desorption tube in a heatable jacket. In the catalogue description, Supelco states:
The use of steam distillation, as noted by Supelco, mandates procedures for the removal of substantial amounts of water which is carried overhead. The use of the adsorbent may still enable a substantial amount of water to be retained. Moreover, the adsorbent can affect the quality of the analysis since different analytes may interact with the adsorbent differently. Supelco presented a paper at the 40th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy held in Atlanta, Ga., during the week of Mar. 6-10, 1989 (Betz, et al., "Utilization of Multi-Bed, Carbon-Based Absorbent Tubes for Adsorption and Subsequent Thermal Desorption of Volatile, Semivolatile and Nonvolatile Organic Compounds Sparged from Environmental Samples".) According to the authors:
Work reported by R. Lemlich, Editor, Adsorptive Bubble Operation Techniques; Academic Press, New York, N.Y. (1972) relates that, due in part to surface energy considerations, compounds such as organic compounds tend to be at a higher concentration at the interface of an aqueous medium and a gas than in the aqueous medium as a whole. By bubbling a gas through the aqueous medium, organic compounds tend toward the interface, and when the bubble breaks the surface, it carries with it some of the aqueous medium surrounding the bubble. The organic compound may be at the interface or in the vapor space in the bubble.
This phenomenon has been referred to as the "adsorptive bubble separation" technique and can be found in nature, e.g., in sea foam and bubbling marshes, as well as in everyday life. Lemlich and coworkers studied the phenomenon using crystal violet chloride as a solute in water and much work was done studying the effect of vessel height, diameter, bubble size and retention on the separation. Lemlich noted at page 141 of the above book that chemical additives can enhance separation, e.g., replacing distilled water with tap water in the crystal violet chloride/water system increased the separation ratio fivefold and the addition of sodium sulfate increased the separation ratio to more than 50. At page 142, Lemlich notes that the addition of certain volatile organic compounds to the gas improves the selectivity of separation.
The adsorption bubble separation technique differs from flotation separation techniques used, for instance, in the paper and mining industries. In the flotation separation technique, bubbles of gas are used to bring a disparate phase, e.g., solids, to the surface of a liquid (see, for instance U.S. Pat. No. 1,069,169). Foams may be used to stabilize the disparate phase.
U.S. Pat. No. 4,314,906 discloses a water purification technique in which water is chlorinated, halogenated organic compounds are produced as a result of the chlorination, and then the halogenated organic compounds are removed by aeration. A carbon filter is used by the patentees to remove any trace amounts of halogenated organic compounds remaining in the water after aeration. The concept of reacting chemical species in liquids and then removing them by, e.g., aeration, or by gas evolution, is also used in ozonation processes such as disclosed in U.S. Pat. Nos. 4,437,999; 4,512,900 and 4,591,444. In, for instance, U.S. Pat. No. 4,437,999, ozone is sparged through a liquid containing organic resin or biological matter while subjecting the liquid to ultraviolet light. The organic material reacts with the ozone to produce water and carbon dioxide. At column 4, lines 48, et seq., the patentees state:
The patentees then suggest that the rate of evaporation of the water be equivalent to the rate of production of water through the oxidation reactions.
In U.S. Pat. No. 4,512,900, the patentees note that in the ozone/ultraviolet light purification method disclosed in U.S. Pat. No. 4,289,594, the presence of surfactants in the liquid caused foaming. Hence, initially the ozone was bubbled through slowly (with the surfactants forming a film of liquid surrounding the bubbles) until the ozone had oxidized the surfactant. Then more vigorous ozonation could occur. The patentees disclose pretreating the liquid with hydrogen peroxide to eliminate undue amounts of surfactant prior to the ozone/ultraviolet light treatment.
U.S. Pat. No. 4,582,629 discloses the use of microwave energy to separate oil and water emulsions. The patentee states that the microwave treatment can be used in conjunction with the separating and heating devices such as skimmers, gun barrel treaters, heater treaters, and the like. U.S. Pat. No. 4,421,651 discloses energizing a molecular sieve adsorbent column with directionally applied microwave energy to produce a mixed gas and liquid effluent. The gas is said to promote the discharge of effluent from the column bottom.